Introduction to Energy Harvesting Transducers and Their Power Conditioning Circuits

Energy harvesters are used to scavenge energy from the ambient, such as light, heat, and vibration, to power wireless sensor nodes. However, power conditioning circuits are needed to maximize conversion efficiency and convert unstable input voltages, either DC or AC, to stable DC output voltages that can be used for sensors or wireless transmitters. The available ambient energy depends on the applications, but each energy source has its own advantages and limitations. Light can be converted to DC electricity through photovoltaic (PV) principle, heat can be converted to DC through thermoelectricity, and vibration energy can be harvested through piezoelectricity or electromagnetic induction mechanism. While light is the most pervasive and has excellent energy density outdoors, its energy density is much lower indoors, and it requires large surface area that can be prohibitive for some applications. Thermoelectric harvester can generate electricity through temperature differences, but in many cases, it would require heat sink to reduce external thermal resistance to build enough temperature difference across the device. It is quiet and reliable with no moving parts. Vibration is pervasive, but vibrational harvester has narrow bandwidth, and its efficiency can drop significantly once the external vibration frequency deviates from resonant frequency for the harvesters. From power conditioning point of view, PV requires maximum power point tracking which itself will consume some currents. Output voltages from the thermoelectric harvester are usually small with limited temperature differences, so step-up would be needed. Load impedance needs to match the internal impedance for maximum power, and output AC output from vibration harvesters need to be rectified. In this chapter, we will go over the common energy harvesters and their power conditioning circuits.